Dinoprost tromethamine, a compound marketed under the trademark Lutalyse.RTM., is described and claimed in U.S. Pat. No. 3,917,864 (incorporated herein by reference). When properly administered this compound is able to induce regression of the corpora lutea of many mammals, especially farm animals such as horses, cows and pigs. The active ingredient is frequently formulated as a solution intended for administration by injection. Here we present a surprising and new formulation of dinoprost tromethamine solution for injection that provides advantages over currently available formulations. This new method of formulation appears suitable for dinoprost tromethamine and other prostaglandin drugs.